2022-05-26 18:27:56

by Matthew Wilcox

Subject: Re: [PATCH] mm/page_table_check: fix accessing unmapped ptep

On Thu, May 26, 2022 at 07:33:50PM +0800, Miaohe Lin wrote:
> ptep is unmapped too early, so ptep will be accessed while it's unmapped.
> Fix it by deferring pte_unmap() until page table checking is done.
>
> Fixes: 80110bbfbba6 ("mm/page_table_check: check entries at pmd levels")
> Signed-off-by: Miaohe Lin <[email protected]>
> ---
> mm/page_table_check.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index 3692bea2ea2c..971c3129b0e3 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -234,11 +234,11 @@ void __page_table_check_pte_clear_range(struct mm_struct *mm,
> pte_t *ptep = pte_offset_map(&pmd, addr);
> unsigned long i;
>
> - pte_unmap(ptep);
> for (i = 0; i < PTRS_PER_PTE; i++) {
> __page_table_check_pte_clear(mm, addr, *ptep);
> addr += PAGE_SIZE;
> ptep++;
> }
> + pte_unmap(ptep);

But ptep was mutated in the loop. So surely this needs to be:

pte_unmap(ptep - PTRS_PER_PTE);

or you'll be unmapping the wrong page.

2022-05-28 19:02:33

by Pasha Tatashin

Subject: Re: [PATCH] mm/page_table_check: fix accessing unmapped ptep

On Thu, May 26, 2022 at 9:34 AM Matthew Wilcox <[email protected]> wrote:
>
> On Thu, May 26, 2022 at 07:33:50PM +0800, Miaohe Lin wrote:
> > ptep is unmapped too early, so ptep will be accessed while it's unmapped.
> > Fix it by deferring pte_unmap() until page table checking is done.
> >
> > Fixes: 80110bbfbba6 ("mm/page_table_check: check entries at pmd levels")
> > Signed-off-by: Miaohe Lin <[email protected]>
> > ---
> > mm/page_table_check.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> > index 3692bea2ea2c..971c3129b0e3 100644
> > --- a/mm/page_table_check.c
> > +++ b/mm/page_table_check.c
> > @@ -234,11 +234,11 @@ void __page_table_check_pte_clear_range(struct mm_struct *mm,
> > pte_t *ptep = pte_offset_map(&pmd, addr);
> > unsigned long i;
> >
> > - pte_unmap(ptep);
> > for (i = 0; i < PTRS_PER_PTE; i++) {
> > __page_table_check_pte_clear(mm, addr, *ptep);
> > addr += PAGE_SIZE;
> > ptep++;
> > }
> > + pte_unmap(ptep);
>
> But ptep was mutated in the loop. So surely this needs to be:
>
> pte_unmap(ptep - PTRS_PER_PTE);
>
> or you'll be unmapping the wrong page.

Right, thank you Matthew.

Miaohe, please store the ptep, or maybe drop this patch entirely.

Thanks,
Pasha